Treatment of rubber



Patented Apr. 23, 1946 PATENT ore 2399,15 IC E "rRnA'rMENT OFRUBBER IPaul Stamberger, Philadelphia,

Roderick Koenig Eskew, Glenside, and Roy S. Hanslick, Philadelphia, Pa.,'assignors to the United States of America, as represented by theSecretary-of Agriculture No Drawing. Application June 15, Serial No.540,500

3 Claims. (01. 260-817) (Granted under the act or March "a, 1083.;

v amended April 30, 1928; 3700. G. 757) This application is made underthe actof March 3, 1883, as amended by the act-of April 30, 1928, andthe invention herein described, iii-patented, may be manufactured andused by or for the Government of the United States of America forgovernmental purposes'without the payment tons of any royalty thereon.

The present invention relates to a treatment of raw resinousrubberwhich-has been recovered from rubber-bearing plants, such as guayule andRussian dandelion.

An object: of the invention is to purify the raw rubber, resulting in arubber which is relatively free from residual plant tissues. are toimprove the physical propertiesof the vulcanizates made from thepurified rubber, to increase the rubber hydrocarbon content, and toimprove the keeping and aging qualities of the purified rubber.

Rubber obtained from the plants by disintegrating the rubber-bearingtissue by mechanical means, for example, by pebble milling, iscontaminated with remnants of planttissues. The rub- Other. objects heris conventionally freed from the larger portion of the disintegratedplant tissue by flotation. In order to waterlog the cork in theflotation process, it is customary to subject the contaminated rubher toa hydrostatic pressure of about 250 lbs. per

square inch at a temperature of about 200 F. In the subsequentflotation, the water logged cork sinks and the raw rubber, in the formof a tangled, wormy mass, floats. Rubber obtained by this treatmentcontains a considerable amount of water (50 percent to 60 percent),andrequires drying at an elevated temperature in vacuum or in circulatedair. The amount of plant impurities present in this rubber is yet veryhigh. For

example, quayule rubber obtained by-such'process contains 10 percent to.15 percent residual plant debris. Consequently, unless the debris isremoved, the tensile strength and otherphysical propertiesof the rubberare impaired.

y Wehave discovered that the raw resinous rubbe! can be very eflectively freed from this residual plant debris by treatment with analkali metal hydroxide solution, for example, by heating with a diluteaqueous (1 percentto .10 percent) NaOH solution, followed by removingthe liquid and dryingthe rubber. The purified rubber obtained in thismanner-has a considerably reduced content of the residual debris,andvulcanized compounds preparedfrom it have improved physical prop-However, we have found that the tenderties. ency for deterioration ondrying and. storing of the alkaline treated rubber makes a practicalapplication of such process of little value. We

attribute this deterioration to thesmall amount of residual alkali whichis strongly retained by soft and any;

the rubber and cannot be removed even by pro longedwater washing. I l-Thisinvention isbased'on the discovery that neutralization of. thisresidualalkali ina particular manner and with certain classes of acidswill eliminate, thistendency for rapid deterioration,.and thatthisneutralization when combined with the alkali treatment already mentionedyields a product with greatly improved properties. We have also foundthat an additional reduction inthe plant debris can be efiected if theneutralization process is combined with-a scrubbing operation, andthat-thequantity, of debriscan be more effectively reduced by usingacids,.the alkali salts of which form detergents during'theneutralization and scrubbing process.

According .to our invention, the wet raw rubber' obtained-from theflotation process above described is placed in 'a container with adilute alkali metalhydroxide solution and heated, preferably from about30 minutes to minutes, at a temperature of about from 180 to 212 F. Theamount of solution used for'such a treatment should be at leastsufficient to cover the rubber and to make stirring and mixing of themass possible. An equal quantity-0f Wet crude rubber and solution byweight is satisfactory. The strength of the solution canvary within awide range, for example, about from 0.5 percent to 20 percent, butaboutfrom 0.5 percent to 5 percent is a preferable concentration. The lowerthe concentration, however, the longer the time and also the higher thetemperature required in -the treat ment. At the end of the'treatment,the darkcolored caustic solution is separated from the mass by flotationand "screening, after which the screened rubber can be washed'withwater.

, If at this stage the rubber is dried, at elevated temperature of aboutfrom to 180 F., it will soon loseitsflrniness and elasticity, becoming,t'h jsho e th su signs o deterioration of'rubber: Without heating inthe drying operation, the same deterioration will be observed duringstorage at room temperature within from 10 days to 60 days. The additionof anage resister (antioxidant) does not effectively alter thisbehavior.

Attempts to remove the residual alkali by wash- 1 ing with 100 to'1',000times the quantity of water originally used does not change the behavioron drying and storing. .We attribute this fact to the porous structureot the-rubberand it tendencytoretain alkali.-

According tothis invention, an acid of definite nature is brought intointimate contact Withthis porous structure, whereby the alkali isneutralized. The acid used for this neutralization is of such, naturethat either it is removed in the drying process by evaporation, or if itremains with the rubber it has no harmful effect thereon.

Formic acid is an'example of volatile acid-which minutesat 170 Fflwith2,000 parts'i'of a 0.1

is removed by evaporation during the drying proc percent solution. offormic acid, the details of ess, and good results are obtained with it,How: the treatment being similar to those of Example ever, since itsalkali salts have no detergent ac This reduced the debris fraction to2.60 portion, the maximum reduction in impurities is not 5 fienti? Therubber 111955 S than The obtained. i ii =1 purified rubber;showed notendency to become Certain water-insoluble acids, for example;-i c y ning.

higher fatty acids, are quitesatisfactoryacids or h treatment-0f thisInvention can also be the type which are not removed in the drying"combined" Wi j leresmatmg treatment as process. Water-soluble acidscannot be used, bem Shwn by following example' cause their presence inthe rubber will retar djyuly i Emmple III canization and have adeteriorating efiect on the final quality of the vulcanized rubber. if:s Asafirst case, guamile raw rubber comparable The best results areobtained by use ofha 'fine that used in Example I was extracted atdispersion, suspension or emulsion of'a water-inf wyt'editempemture witha Solution of Sodium hyof perfectingthe contact is'by scrubbing therubbe'camed out soluble acid which forms with the alkali metal 'f inethyl 9h the stmultaneous. hydroxide an alkali salt (soap-forming acid)with 9 "i debns a extract? the m the properties of a detergent: Thehigher. fatty by h l The t' r -F lb" acids, for example, palmitic,stearicflauric and bar W illehneutrahzed by l i oleic acid, and th like,are examplesof such n as descnbgd m Example soap-forming acids. Thewater dispersion, sus- 1 5 9 s a Second first to extract pension oremulsion should be sumciently fine the resins w1th the solvent andthento heat the d str bution so that the alkali-treated rubber can extractedrubbir t P hydmxide 9 be brought into intimate contact'with the acidic FPl i fi ms operation. a material in the suspension A preferable methodwatendispersion ofz'a soap forming fatty acid can her with thesuspension in a pebble mill, prefer- In this manner a lengthy Waterwashing ably for about from 5 minutes to 20'minutes at a replacgd by ,5neutmfimfibnlpmcsstemperature of approximately '140" to 180 F, PH of thewater extract of such a neutralized The-quantity of aciddispersionshould be such Solution was found to be lower than the'pH ofth t an effective scrubbing operation is possible, the'WatePWa-Shediubberialthwgll in both'cgses one part of the wetalkali-treated rubberto two washmg twatergwas nearly neutral at parts of the dispersion beinga favorable. ratio. completifln of the-washing.'- M 1 The quantity ofacid should be at leasbsumcient The following table illustrates theadvantages that the pH f the Humid .aftelrthe scrubbing of such anadditional treatment-of the rubber Operation win be mm. the neutralpointy indicat with a-stearic acid dispersion after deresination inethat the alkali i ful 1 However, with a 2 percent NaOH solution in ethylalcohol, it is advisable to use an excess of. insoluble acid theextracted rubber having been pebble-mill for the neutralization. f fscrubbed for ZO-minutesat ;170' F.--with' -the The'following examplesillustrate the proces in stearicj acid dispersionl greater detail. a a aI :1

1000 partsof wet guayule raw rubber obtained i b Debris v i extract oiiRcsinsystem'm s by flotation as described above and containing e r b r ocenhecid, percent of water was boiled for 30 minutes with 1: after 2,000parts by weight of a 3 percent sodium hymm; droxide solution. Thesolution was then drained j,,,,,,,,1,;,,,,,, 7 3, $5 ofi to remove asubstantial part thereof and the After H a wet rubber was pebble-millscrubbed for 10min- 5o fi i j' 1' 9 "1 7 utas at 170 F. with 2,000.parts of a water dispermaltreatment; I. ,1 S1011 Containing 2 Prdnfi Sacid. h mass 7.3] 8.1 2.68 i a 1,70

was t en screened, washed with water and dried in the usual manner toproduce the purified rubber. 'The'im'proved aging properties of thevulcan- The fiecb of this treatmentgon the debris fragizate' compoundedfrom the purified rubber of the rich and on the tensile streneth of thevulcanizate tableaboveare shown m the followmg teblei compounded fromthe purified rubber is shown in I I i 1 the following table. Y

r v A, y p p l hysicalf ig ites;

' 60 Physical test before agng or a V Debris, Tensile 5mm V, n8 V G and300 lbs. Guayuic rubber U fraction, ofthevulcaniz te, 1 V r .9 i 5percent' lbspersqdn. sure Before t t t Q- J' h '1 bptimun} optimum -VAfter treatment; %,20 af f n r I a A. -:i tensfle tensile Alter NaOH andstearic acid treat- 65 strength Strength i ments 3 m0 a .of'the tion,.-.oitho l. tmm, I x Y Y vulcamzate, per cent vulcanizate, percent I 1lbs. pg: lbs-.per '51 The insoluble fatty acid can be replacedwith 1 9"sqimiv a water-soluble, volatile, organic acid which will v W. '3, 0' 922, o' 2a evaporate on drying, as for example, formic acid, 70 iiigl ftits m '9 alcohol treatment as shown inthe i'ollowmg example. muowedbystemI Example I! acid treatment.- 3,110 i 800 3, 650 740 The raw rubbertreated with caustlc as-de- V f J scribed inExample Iwas pebble-millscrubber for 75 In compounding the rubber of thethree pre cedingexamples, A. S. T. M. standards with the following formula were used:

Parts Dry rubber 100.0 Zinc oxide 5.0 Sulphur 3.5 Mercaptobenzothiazol1.0 Stearic acid 1.5

Example IV Substantially the same results were obtained with rubberprocessed from kok-saghyz (Russian dandelion) roots. The rubber obtainedwas treated with a 3 percent NaOH solution in water in a manneridentical to the treatment in Example I. The results are given in thefollowing The formula used for compounding the rubber in this examplewas as follows:

Parts Rubber 100.0 Zinc oxide 5.0 Sulphur 3.5 Mercaptobenzothiazol 0.5Stearic acid 4.0

The improvement in the overall properties of the rubber treatedaccording to the present invention was also obtained with landolphiarubber, as demonstrated by the following example:

Example V Raw rubber was obtained from the roots of the plant bymechanical means in a manner similar to that for obtaining guayulerubber. Certain plant constituents remaining in this raw rubber have avery pronounced retarding effect on the vulcanization of the rubbercompound, and ordinarily it is very diflicult to obtain a useful producttherefrom. However, these constituents, as well as other plant debris,are removed by treatment with sodium hydroxide solution and subsequentneutralization with stearic acid by scrubbing in a manner similar tothat of Example I.

Such a purified rubber shows normal behavior on vulcanization, inaddition to the improvement in purity. The results of the chemicalanalysis and the improvement in the physical properties of thevulcanizates and in the rate of cure are given in the following table:

ea Debris 0 1m g I Elonga- Landolphia rubber fraction. um cure of tionpercent tlmeandtemp' 21 per ceiit sq. in.

Before treatment-.- 5.77 18(2l7 1 nBi n. at 1,730 970 After NaOH and1.47 20min.at274 3,640 720 stearic acid treat- 13 ments.

The wet purified rubber for this experiment was dried at 140 F. invacuum before compounding. For preparing the compound for vulcanizationand physical tests, the following formula was used:

Parts Rubber 100.0 Zinc oxide 5.0 Sulphur 3.5 Mercaptobenzothiazol 1.0Stearic acid 1.5

The pH of the water extract of the combined caustic and stearic acidtreated rubber was 7.2.

Having thus described the invention, What is claimed is:

1. A process for purifying raw resinous rubber selected from the groupconsisting of guayule and dandelion comprising treating the raw resinousrubber by mixing it with a hot alkali metal hydroxide solution, removinga substantial part of the solution from the treated rubber, thenscrubbing the treated rubber with a hot Water dispersion of awater-insoluble higher fatty acid Which forms a soap with the alkalimetal hydroxide, the quantity of acid used being at least sufficient toneutralize the residual alkali retained by the treated rubber, and thenWashing and drying the treated rubber.

2. The process of claim 1, wherein the acid is stearic acid.

3. A process for purifying raw resinous rubber selected from the groupconsisting of guayule and dandelion comprising extracting the resinswith ethyl alcohol and treating the raw rubber by mixing it with a hotalkali metal hydroxide solution, removing a substantial part of thesolution from the treated rubber, then scrubbing the treated rubber witha hot water dispersion of a Waterinsoluble higher fatty acid which formsa soap with the alkali metal hydroxide, the quantity of acid used beingat least suflicient to neutralize the residual alkali retained by thetreated rubber, and then washing and drying the treated rubber.

PAUL STAMBERGER. RODERICK KOENIG ESKEW. ROY S. HANSLICK.

